llama : rename ctx to user_data in progress_callback (#7045)

* llama : rename ctx to user_data in progress_callback

This commit renames the `ctx` parameter to `user_data` in the
`llama_progress_callback` typedef.

The motivation for this is that other callbacks use `user_data` or
`data`, and using `ctx` in this case might be confusing as it could be
confused with `llama_context`.

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Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>